I see that REI carries the Exped and probably the others. They have a generous return policy. As you have plenty of time, order all three and take a good look at each. Check out field repair potential. Return the two that don't cut it for you.
I seriously doubt that the synthetic fill material would be affected by heat/humidity. Down, yes.
I've used a full size Thermarest for nearly four years and about 100 nights and have done fine on it. Packs 8x10". Carry it and the sleeping bag on my rear rack in a dry sack.